1. Field of the Invention
The present invention relates to a lens. More particularly, the present invention relates to a fixed-focus lens.
2. Description of Related Art
With the development of modern video technology, image apparatuses such as digital video cameras (DVC) and digital cameras (DC) are widely used in a variety of fields. One of the core elements of the image apparatuses is lens, which is used for forming clear images on a screen or a charge coupled device (CCD). When the image apparatuses are applied in some specific fields, wide angle lenses are used to widen the field of view (FOV). For example, the wide angle lenses are used in car reverse cameras or driving assist cameras. Generally speaking, the horizontal FOV of wide angle lenses for cars is about 100 degrees. In addition, the f-number of normal wide angle lenses for cars is about 2.8. Furthermore, the length and volume of wide angle lenses for cars must be small enough to be mounted in bumpers or at other positions of the cars.
Referring to FIG. 1, a wide angle lens 100 disclosed in U.S. Pat. No. 6,853,495 includes two lens groups 110 and 120 arranged in sequence from an object side to an image side. The refractive powers of the two lens groups 110 and 120 are negative and positive, respectively. The lens group 110 consists of two lenses 112 and 114. The refractive powers of the lenses 112 and 114 are both negative, and the lens 114 is an aspheric lens. The lens group 120 consists of five lenses 121, 122, 123, 124, and 125, and the refractive powers of the five lenses 121, 122, 123, 124, and 125 are negative, positive, positive, positive, and negative, respectively. As the wide angle lens 100 has an aspheric lens (i.e. the lens 114) that requires high manufacturing cost, the cost of the wide angle lens 100 is high. Moreover, the wide angle lens 100 has seven lenses 112, 114, 121, 122, 123, 124, and 125 altogether, so that the length and the volume of the wide angle lens 100 are relatively great.